The first ammonia synthesis loops all used axial flow converters. Due to the axial flow configuration, larger catalyst particles, typically iron based catalysts, were used (typically 6–12° mm) resulting in pressure drops of 15–20 bar around the synthesis loop and significant rate limitations due to mass transport restrictions.
In the 1950s, steam reforming at pressures of 0.5 to 15 bars was introduced in USA, thereby significantly reducing the capital cost of the plants. Until around 1950, plant capacities were expanded by installation of parallel lines of about 70–120 MTPD (Metric Tons Per Day) units. With a few exceptions, the synthesis process ran at pressures of about 300 to 350 bars.
In the period from 1965 to 1985, the concept of integrated plant design was pioneered. This was achieved through construction of large single-train plants with high degrees of energy integration. This resulted in most new plants being based on steam reforming at pressures of 15–30 bars and in ammonia synthesis loops containing catalyst beds operating around 140–220 bars with iron based catalysts. Use of inherently more active, smaller catalyst particles (down to 1.5–3 mm) was made possible through the invention of radial flow and horizontal converters. This decreased the pressure drop in the synthesis loop to around 9–10 bar and at the same time essentially removed the significant mass-transport-limitations of the reaction rate.
In the period from 1985 until today, larger plants have been constructed with some improvements, which have led to specific energy consumption down to approximately 28 GJ/ton of ammonia product. The first commercial non-iron ammonia synthesis catalyst was introduced in the KAAP process of M. W. Kellogg. The promoted ruthenium catalyst on a special graphitised carbon support was claimed to be significantly more active than the traditional promoted iron catalyst. New significantly improved ruthenium catalysts have been reported. DK patent application No. PA 2000 01052, which is incorporated herein by reference, discloses a barium promoted ruthenium catalyst on magnesium oxide support. Ba—Ru/MgO has been claimed to be more active than the commercial Ru catalyst and it exhibited stable activity for 1000 hours at 750° C. at 50 bars. U.S. patent application Ser. No. 09/692,037, which is incorporated herein by reference, discloses a barium promoted ruthenium catalyst on a boron nitride support, Ba—Ru/BN. This catalyst has unprecedented activity and stability and was developed through insight into both the optimal ruthenium crystal size and the influence of the support on the catalytic activity.
Boron nitride (occasionally known as “white graphite”) is a very attractive support material for ruthenium-based ammonia synthesis catalyst. Boron nitride is iso-electronic with carbon, and boron nitride exists just like carbon in several allotropic forms. It has almost the same structure as graphite, except for a different stacking of the individual layers, but it is completely stable towards hydrogenation under all conditions relevant to industrial ammonia synthesis. At the same time, boron nitride is known for its high temperature resistance.
The Ba—Ru/BN catalyst has proved completely stable in 5000 hours operation at 100 bar and 550° C. in an equilibrated 3:1 dihydrogen/dinitrogen mixture. In FIG. 1, the activity and stability of this catalyst is compared to a similar catalyst supported on high surface area graphite. The boron nitride-supported-catalyst is stable also at significantly higher pressures and temperatures.
The activities of the Ru/BN and Ru/C catalysts are measured at 400° C. Ru/BN is aged at 550° C. and Ru/graphite at 450° C.
Ba—Ru/BN exhibits the same reaction kinetics as barium promoted ruthenium on a carbon support Ba—Ru/C. Compared to promoted iron catalysts this means less inhibition by ammonia, lower dihydrogen reaction order and higher activation energy.
The choice of front-end, ammonia synthesis converter and loop configuration has been changed to allow reduced specific investments and to lower the energy consumption. Furthermore, the plant capacities have increased.
Table 1 shows a comparison between various known ammonia synthesis loop configurations.
TABLE 1Comparison between various known Ammonia Synthesis LoopConfigurations1st generation2nd generationHistoricalintegrated plantsintegrated plantsCapacity, MTPD100100020002000Synthesis CatalystFeFeFe/RuFeConverter TypeTVA12-bed radial4-bed3-bedradialradialLoop Pressure,33022090140barSyngas compressor2700154001430019500power, kWhRefrigeration3503000115007700compressor power,kWhCooling water850320082006400consumption, m3/hMake-up gas1263131pressure, bar1Counter-current type of converter.
Since there is still a demand for increased plant capacities, it is important to be able to build even larger plants. This requires an increased pressure in the synthesis loop, e.g. 200 bars simply to reduce the equipment and pipe sizes. Utilising more active catalysts would also assist in reducing equipment sizes. The Ru catalyst on the BN support is perfectly suited for these conditions since it is completely stable towards hydrogenation, which could be a severe problem with carbon supported catalysts at the higher dihydrogen pressures.
Since the current world scale grass root plants provide approximately 2,000 MTPD (2,205 STPD (Short Tons Per Day)) of ammonia, it would be desirable if plants producing above 4,500 MTPD (4,961 STPD) could be built. This is due to the investment per ton of ammonia being approximately 20% lower in such a large plant.
It appears that with the currently available technology, the capacity limitation is around 3,000 MTPD (3,308 STPD) of ammonia.
These problems are, however, solved by the process and catalyst of the invention herein.